1. Field of the Invention
The present invention relates to electrophotographic or xerographic copiers, printers or the like. More particularly, the present invention relates to apparatus and methods for controlling the toner fusing structure in a xerographic or electrophotographic machine so as to improve the quality of the fused toner on produced copy sheets. The present invention is applicable to a wide variety of such toner fusing devices including hot roll fusers, flash fusers, or the like.
2. Description of the Prior Art
The effect of the fuser upon the quality of the final copy in a xerographic or electrophotographic copier/printer environment is well known. If the temperature of such a fusing device is not carefully controlled, the toner temporarily adhering to the copy sheets as it enters the fuser either fails to properly fuse into the substrate thus causing smearing risk on the final copy, or scorching of the copy sheet results if the fuser temperature is allowed to surpass a maximum.
Various arrangements using a single thermistor to switch to different temperature set points for fuser heater control as a function of the conditions of the machine operation are known. Examples are shown in U.S. Pat. Nos. 4,318,612 by Brannan et al. and 4,145,599 by Sakurai et al. In such devices, the reference voltage associated with a bridge circuit that ultimately controls power application to the fuser heaters is effectively driven by the temperature sensors. U.S. Pat. No. 4,001,545 by Wada et al. uses multiple thermistors, each placed directly on the hot roll, as is well known in the art, but also each used to separately control different heater elements to contour the heat applied to the hot roll.
The use of two thermostats that are serially coupled to the heater element with a relay arranged to cut in the higher temperature thermostat when the copy operation is selected so that serial thermostat operation is realized is shown in U.S. Pat. No. 3,505,497 by Lawes et al.
United Kingdom Patent No. 1,092,164 shows three thermostats for high and low temperature control as well as an intermediate temperature control of an incubator-type device.
United Kingdom Patent No. 988,372 uses dual thermistors coupled into separate legs of a bridge for a freeze-dry type of control.
Japanese patent application No. 55-157096 filed Nov. 10, 1980 (published as 57-81278(A) on May 21, 1982), as understood, suggests using one temperature sensor to completely remove power from the fuser when its environment exceeds a maximum level, and a separate temperature sensor to control heat application to the fuser by sensing the fuser temperature directly. Thus, the Japanese application does not teach improvement of copy sheet quality through fusing temperature control, but is merely a thermal breaker type of operation.